Two main products are obtained from the hydrolysis of starch: Sweeteners and dextrins.
The first preparation of dextrose in 1811 led to the development of the corn-sugar industry, whose first manufacturing began in 1872 with the industrial production of liquid glucose.
Corn syrups are hydrolysis products of corn starch, containing dextrose, maltose, higher saccharides and water. Corn starch, and for this matter any other starch consists of a chain of D-glucopyranosyl or dextrose units whose number can vary from 250 to over 1000 units per molecule.
Dextrose is the ultimate depolymerization product of starch.
With higher degrees of polymerization there is maltose, composed of 2 units of dextrose, the higher sugars composed of 3 or 4 units, and finally the dextrins with 5 or more units.
The more converted or depolymerized is the starch, the sweeter is the product. Thus, dextrose is the sweetest of all, while dextrins are nearly tasteless.
On the other hand, higher polymerization yields more viscous products. Therefore dextrins are often chosen to impart viscosity or "body" to syrups.
Besides being used as additives for syrup, dextrins are valuable products on their own, being available in more than 100 different blends and used to make a great number of different gums, pastes and adhesives.
Starch depolymerization can be carried out by 2 distinct techniques: Acid hydrolysis and enzymatic hydrolysis respectively.
Corn syrup can be prepared by straight acid conversion, heating an acidified starch-water slurry with steam via a batch method as described by Hagen in U.S. Pat. Nos. 1,927,312, Sept. 19, 1933 and 1,928,891, Oct. 3, 1933. Continuous straight acid conversion methods have been described by Horesi in U.S. Pat. No. 2,359,763, Oct. 10, 1944, Hughes in U.S. Pat. No. 4,221,609, Sept. 9, 1980 and Church in U.S. Pat. No. 4,318,748, Mar. 9, 1982 among others. Corn syrups can also be prepared by enzymatic routes, treating starch with enzymes such as alpha-amylase.
Both methods have their advantages and shortcomings. The acid method is simple, but lacks flexibility, and usually has to compromise between incomplete depolymerization or degradation products. The enzymatic process is more complicated and time consuming. It also has to deal with difficulties in breaking certain chemical linkages and presents the possibility of partial repolymerization, as well as high sensitivity to impurities in the substract.
More commonly, both methods (acid and enzymatic) are combined so that acid pre-hydrolysis is followed to completion by the use of enzymes.
All the actual commercial methods of starch depolymerization by acid hydrolysis contain a step in which an acidified water slurry of starch is heated. During that heating period, the slurry goes through a physical state called "pasted" or "gelatinized" state, therefore acquiring a highly viscous consistence. This usually happens when the process temperature reaches around 80.degree. C. However, the hydrolysis reaction takes place very slowly at this temperature. As it is the case for any chemical reaction, starch hydrolysis can be accelerated dramatically with temperature increase. This is highly desirable because it allows the reaction to be carried out in much smaller reactors, therefore cutting costs and simplifying the whole process. The limit on this downsizing is established by how high temperature can be raised, and this in turn depends on how rapidly the product can be heated and cooled again before degradation can set in.
Once starch is heated to 80.degree. C. and becomes gelatinized, it then becomes difficult to heat rapidly because viscosity suppresses convection currents, which are essential for fast heat transfer.
There are three approaches actually used to attempt circumvention of this problem.
First, there is the option of working at lower temperatures, which is usually the case for batch procedures. It is associated with low mixing speeds and slow heat transfer. The resulting product has an acceptable quality level but the process has the drawbacks of requiring bulky equipment and long retention times. Long retention times are particularly undesirable because they obviously increase the price of a processing plant.
The second option consists of heating the heat transfer surfaces beyond recommendable limits in order to speed-up the reaction. In this case, the reaction rate is increased but the product loses quality due to uneven heating. In fact, uneven temperatures throughout the reactants are objectionable because in acid starch hydrolysis the reaction does not stop when dextrose is produced, but continues towards highly unwanted degradation products. Thus, before all the starch is hydrolysed, part of the finished product will undergo thermal degradation.
Finally, high shear forces can be used to impart forced convection to these "pasted" slurries as described in U.S. Pat. No. 4,221,609 by Hughes, in which an acidified starch slurry is forced with a pump through a pipe system heated from the outside. Such a system allows to work at higher temperatures and to shorten retention times to a few minutes. However, where shear force is used to pump viscous material through a pipe, a laminar flow regime with its inherently parabolic profile of velocities is observed. In other words, material in the central part of the pipe flows faster than material at the periphery, therefore leading the unequal residence times for different parts of the reactants.
In order to overcome these problems, it would be highly desirable to provide a starch depolymerization technique that would allow: elimination of the viscous intermediate state of starch containing slurries, reduction of reaction time from minutes to seconds, reduction of the size and price of equipment, reasonably high operation temperature to allow short residence time but no thermal degradation of the slurry and uniform residence time for all reacting matter passing through the reactor, thus resulting not only in a product possessing superior quality and low production costs, but also greater flexibility by allowing selective production of the needed starch depolymerization product.